The invention relates to a photopolymer formulation comprising matrix polymers, writing monomers and photoinitiators. The invention furthermore relates to the use of the photopolymer formulation for the production of optical elements, in particular for the production of holographic elements and images, a process for the preparation of the photopolymer formulation and a process for exposing holographic media comprising the photopolymer formulation.
Photopolymer formulations of the type mentioned at the outset are known in the prior art. Thus, for example, WO 2008/125229 A1 describes a photopolymer formulation which contains polyurethane-based matrix polymers, an acrylate-based writing monomer and photoinitiators. In the cured state, the writing monomer and the photoinitiators are embedded with spatially isotropic distribution in the polyurethane matrix. The WO document likewise discloses that further components, such as, for example, dibutyl phthalate, a classical plasticizer for industrial plastics, can be added to the photopolymer formulation.
For the uses of photopolymer formulations, the refractive index modulation Δn produced by the holographic exposure in the photopolymer plays the decisive role. During the holographic exposure, the interference field of signal and reference light beam (in the simplest case, that of two plane waves) is mapped by the local photopolymerization of, for example, highly refractive acrylates at sites of high intensity in the interference field into a refractive index grating. The refractive index grating in the photopolymer (the hologram) contains all information of the signal light beam. By illumination of the hologram only with the reference light beam, the signal can then be reconstructed. The strength of the signal reconstructed in this manner in relation to the strength of the incident reference light is referred to as diffraction efficiency, or DE below. In the simplest case of a hologram which forms from the superposition of two plane waves, the DE is the quotient of the intensity of the light diffracted on reconstruction and the sum of the intensities of incident reference light and diffracted light. The higher the DE, the more efficient is a hologram with respect to the quantity of light of the reference light which is required for making the signal visible with a fixed brightness. Highly refractive acrylates are capable of producing diffraction index gratings having a high amplitude between regions with low refractive index and regions of high refractive index and hence of permitting holograms having a high DE and a high Δn in photopolymer formulations. It should be noted that DE is dependent on the product of Δn and the photopolymer layer thickness d. The greater the product, the greater is the possible DE (for reflection holograms). The width of the angle range in which the hologram is visible (reconstructed), for example on monochromatic illumination, depends only on the layer thickness d. On illumination of the hologram with, for example, white light, the width of the spectral range which contributes to the reconstruction of the hologram may likewise be dependent only on the layer thickness d. The smaller d, the greater are the respective acceptance widths. If it is therefore intended to produce bright and readily visible holograms, a high Δn and a small thickness d are desirable, in particular so that DE is as large as possible. This means that the higher Δn, the more latitude is achieved in configuring the layer thickness d for bright holograms without loss of DE. The optimization of Δn is therefore of outstanding importance in the optimization of photopolymer formulations (P. Hariharan, Optical Holography, 2nd Edition, Cambridge University Press, 1996).